This invention is based on the reception and processing, with new means and methods, of signals emitted by an equipment present on board of all aircraft operating in the controlled airspace, i.e. the transponder of the secondary surveillance radar, SSR for short. The invention applies to both transponders with less recent characteristics (Mode A and C, not selective, with 12 bit indication of the target identity—Mode A—and of the barometric altitude—Mode C) and for transponders with more recent characteristics (selective Mode S or selective, with reply code of 112 or 56 bit), as well as for equivalent equipment (called “non-transponder devices”) for vehicles even though not respecting air-worthiness requirements.
In such a context, the present invention refers to a system, internationally called Multilateration (MLAT) whose functional and operational requirements are contained in the following document written from the International Organization Eurocae—The European Organization for Civil Aviation Equipment:    [1] Eurocae ED-117 “Minimum operational performance specifications for Mode S multilateration systems for use in A-SMGCS”, April 2003, available at Eurocae, 17, rue Hamelin, Paris Cedex 16.
An MLAT system has the scope of localization and identification, typically but not exclusively in the airport area, of the aircrafts and the vehicles equipped with an appropriate transponder, more exactly of above-mentioned transponder of the secondary surveillance radar (SSR).
The SSR radar system both of traditional type or “mode A/C” and of selective type or “mode S”, is described in several documents among which the book:    [2] M. Stevens “Secondary surveillance radar”, Artech House, 1988.
A system for control and management of airport traffic that can include Multilateration (MLAT) is internationally known with the acronym A-SMGCS (Advanced-Surface Movement Guidance and Control System). In such a system the present invention, from the applications point of view, is framed. A-SMGCS is described, among the others, in the document:    [3] Eurocae ED-87A “Minimum performance specifications for Advanced-Surface Movement Guidance and Control Systems”, January 2001, available at the Eurocae, 17, rue Hamelin, Paris Cedex 16.
MLAT systems receive and process the replies emitted by SSR transponders, once interrogated, and/or the “spontaneous replies” emitted periodically in the absence of interrogations, and called “squitter”. The processing of such signals, as described in [1] and [2], permits to localize the transponder, and therefore the aircraft or the vehicle carrying this, by means of the measurement of the times that elapse from the emission—by the transponder—of the reply/squitter signal (that thereafter will be called simply “SSR signal” or shortly “signal”) and its reception by MLAT receiving stations (four or more). The measurement of at least three differences of the times of arrival (TDOA technique: Time Difference of Arrival also said Delta-TOA: Difference in Time of Arrival) permits to localize the transponder in the space through intersection of three hyperboloids. Such method has been applied already from the half of the twentieth century to other types of signals, the so-called hyperbolic navigation with classic algorithms described in several works among which:    [4] R. O. Schmidt “A new approach to geometry of range difference location”, IEEE Transactions on Aerospace and Electronic Systems, Vol. AES-8 No. 6, November 1972, pp. 821-835.
Multilateration technique has been the object of many patents, (starting from the ancient U.S. Pat. No. 2,972,742 by Ross) among which:    [5] U.S. Pat. No. 3,659,085 of 25 Apr. 1972: “Computer determining the location of objects in a coordinate system”, inventors: B. And Potter; T. K. Bosworth; J. P. Chisholm; J. A. Cadzow, in which methods are proposed in order to reduce the localization error, in a general context, that is specific neither of the application to SSR signals (A/C or Mode), nor to the use for airport surveillance, and    [6] European Patent Number: EP 0466239 of 15 Jan. 1992: “Device for identifying and localizing transponders”, inventor Frans Herman De Haan (NL), in which the airport is divided in a considerable (100-250) number of regions each of them has at least one, better two, emitters and at least two receivers in order to localize SSR transponder by means of Multilateration, and in which emitters and receivers are connected to a central processing unit.
In today's market there are three different MLAT systems developed respectively by the Companies: ERA (Czech Republic), Sensys (USA) and Thales (Germany) and presented at the specialized International Exhibitions or Fairs, such as the Maastricht Exhibition on Air Traffic Management that is carried out in spring of every year. However, specific patents related to such MLAT systems have not been found. In a generic MLAT system, each station is equipped with a receiver for SSR signals that allows it to measure the time of arrival of each of them. Such measurement can be got, according to two possible and functionally equivalent realizations whose choice depends on the set of operations, as follows. The first solution includes local measurements, therefore executed at each measuring Station, equipped with a precise clock (locked to that one of a master station through radio connection or on cable), while in the second solution the measurements are executed at the central processing station; in this second realization SSR signals are simply transmitted in real time and transparent mode to the central processing, where the measurements of time are executed. The MLAT system developed by ERA uses the second solution, with transmission of the waveform of SSR signals from the receiving stations to the central processing computer, while other two systems (Sensys and Thales) are based, instead, on the second solution and have measuring stations not only receiving but also with processing capacity. All of three systems can be considered “first generation” systems, while the System, equipped with digital transmission and object of the present invention, is a system of second generation for its advanced characteristics well described later.